Acrylic acid is industrially important as a starting material for various synthetic resins, paints, plasticizing agents and the like. In recent years, importance of acrylic acid particularly as a starting material of water absorbent resins gathers weight and the demand therefore tends to increase.
For production of acrylic acid, new processes such as catalytic vapor-phase oxidation of the acrolein which is obtained through dehydration of glycerin from natural sources have been proposed in recent years. However, two-stage catalytic gas-phase oxidation comprising producing acrolein by catalytic gas-phase oxidation of propylene, and producing acrylic acid by catalytic gas-phase oxidation of the resulting acrolein is the most generally used process, which has been widely and industrially worked. Concerning such production process of acrylic acid by two-stage gas-phase catalytic oxidation of propylene, a number of companies have offered various proposals aiming at acrylic acid production in a higher yield stably over a longer period.
For example, Patent Document 1 (U.S. 2004/0015013A1) discloses a process wherein a part of the deteriorated catalyst during the catalytic vapor-phase oxidation is replaced with fresh catalyst, to produce the object product stably over a longer period, without changing the whole catalyst layer. Also Patent Document 2 (JP 2005-95874A) discloses a process wherein the deteriorated catalyst is once discharged from the fixed bed reactor, regenerated, and re-loaded. Patent Documents 3 (JP 6341988)-137755A), 4 (JP 6(1994)-262081A), 5 (JP 2007-509884T), 6 (JP 2007-509864T), 7 (JP 2007-509867T) and 8 (JP 2008-6359A) disclose the processes comprising suspending the reaction temporarily and passing a specific gas through the catalyst layer(s) to regenerate the deteriorated catalyst.
Acrylic acid is currently produced on a scale of several millions of tons per year in the whole world, and the demand therefore has been increasing in recent years particularly as a starting material of water absorbent resins. Still in addition, price of its starting gas is rising drastically. Under the circumstances, even a little improvement (e.g., by 0.1%) in the yield on industrial scale production of acrylic acid will bring a very significant economical merit. While all of the above-enumerated known processes achieve certain improvements in respect of acrylic acid yield or longer operation period as intended, they still leave room for improvement viewed from industrial scale production. For example, the partial catalyst exchange or catalyst regeneration as proposed in the cited patent documents can extend the catalyst life, but require temporary suspension of the reaction during which the production amount of acrylic acid decreases. Thus, economically the processes are not yet fully satisfactory.
This led us to the thought, if a novel process to control the reduction in catalytic activity itself during the reaction could be found, it would enable to extend the duration of the usable period of the catalyst without suspending the reaction, and to enable continuous operation over a prolonged period. Still in addition, we thought that the usable period of the catalyst would be further extended if such a novel process is adopted in combination with the known regeneration treatment as referred to in the above.
Accordingly, therefore, the object of the present invention lies in continuous production of acrolein and/or acrylic acid in high yields over a longer period, by applying such a novel process to industrial scale catalytic gas-phase oxidation of propylene or a propylene-containing gas with molecular oxygen or a molecular oxygen-containing gas, or to industrial scale catalytic gas-phase oxidation of acrolein or an acrolein-containing gas with molecular oxygen or a molecular oxygen-containing gas.